1. Field of the Invention
This invention pertains to an optical fiber device having two optical fibers whose tips are arranged facing each other and are optically coupled, and an optical functional element interposed between the same tips.
2. Description of the Related Art
A wide variety of optical fiber devices consisting in interposing an optical functional element effecting an action corresponding to an objective with respect to propagating light, e.g. a moving mirror, a moving shutter or the like, or a fixed optical filter or the like, in an optical path constituted between the tips of optical fibers whose tips are arranged facing each other, are proposed and put into practical use.
In optical fiber devices like these, reflected return light which is reflected at the end faces and so forth of the optical fibers and is incident again into the optical fibers is a cause of noise. Therefore, in optical fiber devices like these, the reduction of such reflected return light has become an important task.
E.g., in U.S. Pat. No. 6,315,462 (Patent Reference 1), there is described an optical fiber device in which a moving mirror is arranged between the tips of optical fibers in which the tips are arranged facing each other and designed to carry out switching of the optical path by inserting and removing a moving mirror with respect to the optical path. In this Patent Reference 1, there is described forming a reflection prevention film on the optical fiber end faces which should reduce reflected return light at the optical fiber end faces and, further, dipping the optical fiber end faces and the mirror in a refractive index matching agent (matching oil) to eliminate the refractive index difference at the interface.
Also, e.g. in Japanese Patent Application Laid Open No. 2005-37885 (Patent Reference 2), there is described, similarly to Patent Reference 1, performing oblique polishing of the optical fiber end faces which should reduce reflected return light, in an optical fiber device carrying out switching of the optical path by means of a moving mirror.
Further, e.g. in Japanese Patent Application Laid Open No. 1981-39507 (Patent Reference 3), there is described, in an optical fiber connector connecting optical fibers, reducing the end face reflected light, without performing oblique polishing of the optical fiber end faces, by mounting rod lenses respectively to the end faces of the connected optical fibers and adjusting the pitch of the same rod lenses.
Moreover, in optical fiber devices wherein an optical functional element is arranged between the optical fiber tips, by providing a lens on the optical fiber tips, there is brought a degree of freedom in carrying out operations like reducing the spot size of the light radiated on the optical functional element and changing the working distance from the optical fiber tips to the optical functional elements. E.g., in Japanese Patent Application Laid Open No. 2003-43270 (Patent Reference 4), there is described a configuration designed to be able to independently modify both the aforementioned light spot size and working distance by connecting a spacer with a uniform refractive index to the optical fiber end faces and connecting a graded index optical fiber to the other end face of the same spacer.
As mentioned above, as a method for reducing the reflected return light at the optical fiber end faces, there have been the methods of carrying out operations like forming a reflection prevention film on the optical fiber end faces and dipping the optical fiber end faces in a refractive index matching agent (e.g. Patent Reference 1). These are methods of reducing the reflected light itself, but there are limits to the performance of reflection reduction. E.g., even if a reflection prevention film (non-reflective multi-layer film) with a performance which is the highest at today's technical level is used, the reflection attenuation thereof has an upper limit on the order of 45 dB. Moreover, the reflection attenuation in the case of using the method of dipping the optical fiber end faces in a refractive index matching agent is also of the same order.
As against this, the configuration of carrying out oblique polishing of the optical fiber end faces (e.g. Patent Reference 2) is one which suppresses the incidence again of reflected light on the optical fiber, the reflection coupling loss thereof attaining 60 dB. This value is a sufficient value for optical fiber devices. However, for the manufacturing of optical fiber devices with this structure, the manufacturing process called oblique polishing is necessary, so the cost associated with the same polishing step becomes necessary. In addition to this, with this structure, since the light is emitted obliquely from the obliquely polished optical fiber end faces, the design and fiber inspection work of the coupling system corresponding thereto becomes complex and difficult, and also, there is the disadvantage of several constraints arising from the fact that the light is emitted obliquely. In addition, this structure also has the drawback that the efficiency of the transmitted light is inferior compared to the aforementioned method of reducing the reflected light itself.
Moreover, in the structure described in Patent Reference 3, the reflected light is reduced by using a rod lens adjusted to the required pitch, in substitution for the oblique polishing of the optical fiber end faces. However, what is described in Patent Reference 3 is an optical fiber connector, so this structure cannot be applied to an optical fiber device having a structure in which there is provided a gap between optical fiber tips and an optical functional element is interposed in the concerned gap.
Further, as an optical fiber capable of sufficiently reducing the reflected return light, without oblique polishing of the optical fiber end faces, there is the spherically tipped optical fiber in which the core itself of the terminal portion of the optical fiber has been shaped into a spherical surface. However, a spherically tipped optical fiber has a working distance which is exceedingly short and a beam waist diameter of the emitted light which is exceedingly small, so there is the drawback that the freedom in the design of an optical system is small because of these points. In addition, it is not straightforward to form, with accurate control, the three-dimensional shape of such a core. Because of that point, the cost of spherically tipped optical fibers is high.
Moreover, notwithstanding the fact that there is described in Patent Reference 4 an optical fiber terminal structure for which a high degree of freedom can be obtained regarding the light spot size and working distance, there is not in any way a description regarding reducing reflected return light in this Patent Reference 4.